Method for presoldering a contact for an electrical switching device and semi-finished product for use as a contact

ABSTRACT

Contacts such as contact points, contact strips, or contact sections are usually provided with hard solder on a copper-silver basis in the form of a flat solder layer. The contact is then connected over this solder layer with a contact carrier. The solder layer is melted, whereupon the free surface of the solder layer (13, 130) is covered during melting with a material (1, 5, 15) that has no solubility with silver or copper. The material can, during melting of the solder, form a covering (1, 15) beneath the contact (10, 12, 100) provided on its solder side with the solder layer (13, 130) or can also form a covering over the contact (10, 12, 100) provided on its solder side with the solder layer (13, 130). As materials for covering the solder layer (13, 130), high-melting metals, preferably tantalum, or a ceramic are used. In the case of a semi-finished product for use as a contact, the solder layer (13, 130) is in intimate contact over its entire surface with the solder side of the contact (10, 12, 100 ) and has the structure of a melt.

BACKGROUND OF THE INVENTION

The present invention relates to a method for presoldering a contact foran electrical switching device with CuAg-based hard solder, over whichthe contact is connectable with a contact carrier for which purpose thehard solder must be applied as a flat layer, preferably with a definedcontour, for example using a solder foil or the like, with the appliedsolder layer being melted. The term "contact" refers to an individualcontact point, but can also refer to contact strips or sections to beused in cutting individual contact points to size. In addition thepresent invention relates to a semi-finished product for use as acontact in an electrical switching device, especially a contact point,contact strip, or contact section, with at least one contact layer andone solder layer being in largely intimate contact with the solder sideof the contact over its entire area and having the structure of a melt.

Contact points manufactured by molding technology are usually attachedby hard soldering to the contact carriers of switching devices. For thispurpose, the solder is applied as a flat layer to the back of thecontact point or the contact material in the form of a strip, forexample using a solder foil. If the contact is initially in the form ofa contact strip or section, from which contact points are later cut tolength, a solder layer can also be produced initially by roll bonding.In particular, solder foils can be cut to length simply using anultrasound technique to form a solder pad and attached uniformly to thesolder side of contact points, as described in the German PatentDocument No. DE-A-40 24 941. Contacts presoldered in this manner can beprocessed further mostly automatically and attached within the frameworkof an integrated manufacturing process, for example by inductive heatingand soldering to the contact carrier.

In the latter energy-induced process, problems can occur because solderfrom the solder side rises above the narrow sides of the contact pointand gets on the switching surface. Because of such uncontrollableprocesses, the switching behavior of the contacts can be affected in anundesirable manner during the proper use of the switching device.

It is therefore desirable for the solder to be applied in a smooth layerduring presoldering of contacts, with the area of the solder layergenerally being smaller than the area on the solder side of the contact.This is intended to prevent wetting of the narrow sides of the contactwith solder.

Usually, especially for switching devices used in energy technology,contacts with a contact material layer with the silver metal composition(AgMe) or silver metal compound (AgMeV, where V is a compound),especially silver metal oxide (AgMeO) are used. Frequently such contactshave a two-layer structure, i.e. they consist of the contact layeritself and a layer of pure silver on the solder side. A solder suitablefor this purpose is known which is based on copper and silver, whichalso can contain phosphorus in particular.

When, according to the prior art, solder foils are applied to the soldersides of contacts, it may be seen that when the solder melts, the solderpreferably initially melts at the edges of the solder layer and air orgas inclusions between the solder foil and the contact point usuallyturn into a central bubble on the solder side of the contact point. Toeliminate such bubbles which cause problems with subsequent processingof the presoldered contact points, as well as the associatedirregularities on the surface of the solder layer, the solder must beheated until the bubble bursts as a result of the increase in itsinternal pressure, and breaks up. At such high temperatures, the soldergenerally runs onto the contact side of the contact point.

Patent Document No. GB-A-11 62 887 describes a method for continuousfastening of a narrow solder strip to a metal strip, for example acontact surface. The solder strip is briefly melted to form an intimatebond with the metal strip and has the structure of a melt. The shape ofthe solder strip is generally lost in the process, with at least aconvex surface being obtained. U.S. Pat. No. 2,216,510 discloses amethod for manufacturing contacts in which shaped parts are stamped fromsoldered parts and are further processed. The plates, presoldered usingdifferent methods, are then rolled to ensure a flat surface.

SUMMARY OF THE INVENTION

Taking its departure from this state of affairs and the above-describedprior art, the present invention provides a method for presolderingcontact points with which a flat solder layer with a defined thicknessand a sharp edge contour can be applied. In this method, wetting of thecontact side surfaces with solder is particularly to be avoided. At thesame time a semi-finished product is provided for use as contacts inswitching devices.

According to the present invention the applied solder layer is melted,with the free surface of the solder layer being coated during meltingwith a material that has no solubility for silver or copper. This methodcan be used advantageously both for individual contact points and forcontact strips and foils.

According to the present invention, a semi-finished product may beobtained for use as contacts, in which the solder layer is in intimatecontact with the solder side of the contact over its entire surface, andin which the solder layer has the texture of a melt. Advantageously thesolder layer is largely bubble-free on the solder side of the contact orat least has no bubbles whose diameters are larger than the thickness ofthe solder layer. With a two-layer contact point with a silver-metaloxide layer on the contact side and a pure silver layer on the solderside and a copper-silver-hard solder with a phosphorus content, the meltstructure consists of a ternary eutectic and mixed crystals rich incopper or silver. Hence, the use properties of the solder layer inparticular are not adversely affected by dissolved silver.

In the method according to a first embodiment of the present inventionthe material for covering the free surface of the solder layer can forma covering beneath the contact provided on its solder side with thesolder layer. In a second embodiment of the method according to thepresent invention, the material for covering the free surface of thesolder layer can form a covering over the contact provided on its solderside with the solder layer. In this case, the contact is reverselyarranged for the melting process. In both cases the covering can also beformed by a roller rotating around its axis in which a contact strip orsection including the solder strip is continuously guided.

Either a high-melting metal, e.g. tantalum (Ta), molybdenum (Mo), ortungsten (W) or ceramic can be used as a material for forming thecovering. It is important in this regard, in addition to thethermodynamic properties of the material, such as insolubility forcopper and silver, not to permit any or to permit only a very smallwettability of the covering material with liquid solder. It hassurprisingly been found that creep of liquid solder into undesiredplaces on the contact points is prevented. In particular this means thatat least the side of the contact which is opposite the solder layer isfree of solder. In addition, wetting of the narrow sides of the contactwith liquid solder can thus be largely prevented.

The present invention has worked especially well when solderingtwo-layer contact points which consist of a silver-metal oxide layer onthe contact side and a pure silver layer on the solder side and whichare soldered with a solder containing phosphorus, e.g. L-Ag15P. It hasproven to be especially suitable in this regard to perform melting byinductive heating, with locally directed heating being achieved and withthe temperature-time curve being usable for process control. However,furnace heating is also possible if the holding time is chosen to becorrespondingly short. In addition, for rapid, local heating, a laser ora high power lamp can be used as well. In all cases, the melting of thesolder layer advantageously takes place under a protective gas,preferably under noble gas or nitrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details and advantages of the present invention will be apparentfrom the following description of the figures showing embodiments withreference to the drawings, in conjunction with the claims.

FIG. 1 is a first example illustrating the procedure according to thepresent invention with a covering beneath a soldered contact point.

FIG. 2 illustrates the contact point soldered in FIG. 1, in a bottomview.

FIG. 3 is a graph showing temperature as a function of time for a systemaccording to FIG. 1.

FIG. 4 is an example of an alternative procedure to FIG. 1 with acovering for the soldered contact point.

FIG. 5 is an example corresponding to FIG. 1 for manufacturing contactstrips or sections.

FIG. 6 illustrates the microstructure of a solder layer of asemi-finished product for contacts.

DETAILED DESCRIPTION

In FIG. 1, a molded body 1 made of tantalum (Ta) forms a substrate for acontact point 10 to be soldered with a flat layer 13. Substrate 1 hasits top shaped with recesses 2 and 3, with recess 2 matching contactpoint 10 in size and shape and recess 3 matching solder layer 13 in sizeand shape.

Contact point 10 is designed as a two-layer contact point and has alayer 11 made of silver metal oxide as a contact (for example, AgSnO₂Bi₂ O₃ CuO). Since this material cannot be soldered, in the manufactureof contact point 10 which is generally performed using sinteringtechnology, its underside is provided with a pure silver layer 12 whichis readily solderable. Solder layer 13 is applied to pure silver layer12, by means of which contact point 10 can later be attached for use ina contact carrier (not shown) in a switching device. In particular, toform solder layer 13, a foil of a suitable CuAg-based hard solder,especially L-Ag15P (so-called Silfos solder) is used, which has amelting range between 650° and 800° C. and a working temperature ofapproximately 710° C.

A solder foil 13 of this kind can be cut to length in a suitable sizefrom a continuous strip and fitted into recess 3 of substrate 1. Thencontact point 10 is applied with its pure silver layer 12 so that theunderside rests tightly against solder foil 13. To ensure that theunderside rests tightly against solder foil 13, it is important to haveexact dimensional stability and fit of the corresponding recesses 2 and3 in substrate 1.

FIG. 2 illustrates the geometry of the finished, soldered contact point10 in the shape of a rectangle. Pure silver layer 12 has a larger areathan solder layer 13 placed on top of it, with a marginal area that isfree of solder. It is especially important in this connection thatsolder layer 13 have a flat surface, that no central bubble form betweensolder layer 13 and silver layer 12 and that finished solder layer 13 becompletely flat with a constant thickness and sharp contours with adefined geometry. In particular the latter conditions may be neglectedin the normal case when melting a solder foil 13. Then liquid solder canspread out over silver layer 12 without definition, over its edges, andalso cause wetting of the side surfaces of contact point 10 with solder.

Substrate 1 according to FIG. 1 with recesses 2 and 3 avoids the latterundesirable effect. It is important in this regard for the tantalumitself to have virtually no wettability even for liquid solder. Inaddition, tantalum exhibits no solubility for silver or copper. Atantalum phosphite that theoretically forms has a high melting point sothat it is harmless.

In FIG. 1, turns of an inductor 20 are located for melting solder layer13 above contact point 10. This provides rapid and directed heating ofthe system with contact point 10 and solder foil 13. For processcontrol, a temperature sensor 30 is directed at the surface of contactpoint 10. Temperature sensor 30 operates, for example, according to theinfrared radiation principle. In addition, a weight 31 is provided aboveinductor 20 so that the system composed of contact point 10 and solderlayer 13 can be loaded by means of an intermediate element (not shown ingreater detail) in order to ensure good heat transfer between contactpoint 10, solder foil 13, and tantalum substrate 1.

FIG. 3 shows the temperature control in detail. By means of eddycurrents produced in the metal material by inductor 20, which isconnected to a source of alternating current, contact point 11 isinitially heated with a temperature rise that is approximately uniform.When the lower temperature of the melting range of the solder used isreached at t_(A), namely approximately T_(S) =650° C. for Silfos solder,a dip 55 appears in the time-temperature curve 50, since melting heat isrequired and heat equalization takes place between the warmer contactpoint 10 and the colder tantalum substrate 1. Then the system (contactpoint 10-solder layer 13-tantalum substrate 1) warms up at a slowerheating rate. At time t_(E) the upper temperature of the melting range,i.e. the liquidus line in the state diagram, has generally not yet beenreached. However, the working temperature of the solder being usedprevails, and the system is switched off so that the entire system cancool down.

The process involving melting of solder layer 13 can be regulated bymeans of the temperature curve T=f(t). In particular, the rapidtemperature rise is advantageously first achieved in the contact pointby means of inductive heating. Melting of the solder and melting ofsolder layer 13 then takes place in a very short time. In addition,since barriers are formed by shapes 2 and 3 in tantalum substrate 1,practically no solder can reach the side surfaces of contact point 10.

FIG. 4 shows contact point 10 according to FIG. 1 in the reverseorientation. This means that contact 11 is on the underside and the puresilver layer 12 is on the top side. A hard solder foil for formingsolder layer 13 is placed on silver layer 1 and on top of that is acovering 5 with an edge profile 6. The entire system is on a belt 8 thatcan be guided as a belt moving through a heating furnace.

In the system according to FIG. 4, covering 5 of solder layer 13 forms asurface for contact point 10, and in this case is made of ceramic, forexample, which is not wetted by the solder and has no solubility forcopper or silver. It is advantageous in this system for gases to be ableto escape upward through the molten solder.

In FIG. 5, a roller 15 rotatable about its central axis A is provided,serving as a substrate for a contact strip 100. Contact strip 100 isproduced, for example, by continuous casting of a powder blank andconnected by roll bonding with a solder strip 130. Contact strip 100including solder strip 130 can also be made in the form of a shape.Contact points can later be cut to size from such strips or sections foruse as intended. Roller 15 has recesses 22 and 23 in its surface toreceive contact strip 100 and solder strip 130, with an inductor 20being provided according to the embodiment of FIG. 1. Upon rotation ofroller 15, which is composed of high-melting metal or ceramic, thesolder portion located in the active area of inductor 20 is melted intothe form of a flat layer, with the exactly defined shape again beingpreserved laterally.

In FIG. 5, roller 15 is shown as a substrate for continuous contactstrip 100 with solder layer 130. In the reverse arrangement, contactstrip 100 with solder layer 130 can also run beneath roller 15. In bothcases the function of a covering is ensured.

The presoldering process involving melting of solder in all examplestakes place under a protective gas, preferably a noble gas such as argonor helium, or under nitrogen. As a result a semi-finished product isobtained in which the solder layer has the structure of a melt. Sincethe contact is preferably made by powder metallurgy, the contact layer(and in a two-layer structure, its solderable silver layer as well) hasa sintered structure and the solder layer has the structure of a melt. Alight microscope view shows a sharp phase transition between the layers,but a solid material bond is present. There is practically no chance ofharmful dissolution of the silver which is in contact. In particular,the solder surface is then free of defects.

It is clear from FIG. 6 that the molten solder layer is in intimatecontact with the solder side of the contact rand has the structure of amelt. As a result of heating which is only brief, silver is onlyslightly dissolved on the solder side of the contact point. However inthe transition area up to a thickness of a maximum of one-third of thesolder layer, silver dendrites can form. Any gas inclusions that arestill present are so small that they do not adversely affect the surfaceof the solder layer and therefore cause no problems.

Solder layer 13 or 130 in FIG. 6 is largely free of silver dendrites.The structure of the solder layer, according to state diagram Cu-Ag-P,mainly consists of ternary eutectic and also of mixed crystals rich incopper or silver. The precise melting of the solder layer beneath theliquidus line in the state diagram produces the usage properties of thesolder without changing the working temperature.

In the same manner as the method described above, contacts based onsilver-metal, for example silver-nickel, or based on other silver-metalcompounds, can be coated with suitable solder. Instead of a solder foil,solder pastes may be used to form the solder layer. In addition totantalum or ceramic, tungsten or molybdenum or other suitable metals maybe used as material for the covering beneath or on top of the contact.

What is claimed is:
 1. A method for presoldering a contact for anelectrical switching device comprising steps of:connecting the contactwith a contact carrier using CuAg-based hard solder, wherein the hardsolder is applied as a flat solder layer; and melting the applied solderlayer wherein during the melting the free surface of the solder layer iscovered with a material that exhibits no solubility with respect tosilver (Ag) or copper (Cu).
 2. A method according to claim 1, whereinduring the step of melting the applied solder layer, the material thatexhibits no solubility with respect to silver or copper forms a coveringwhich is located beneath the contact on a solder side with the solderlayer.
 3. A method according to claim 2, wherein during the meltingstep, the covering forms a receptacle for the solder layer and thesolder side of the contact.
 4. A method according to claim 1, whereinduring the melting of the solder, the material forms a covering locatedon top of the contact on a solder side with solder layer.
 5. A methodaccording to claim 1, wherein the material for covering the solder layeris a molded body made of a high-melting metal.
 6. A method according toclaim 1, wherein the material for covering the solder layer is a ceramicmolded body roller.
 7. A method according to claim 1, wherein themelting of the solder layer is produced by inductive heating.
 8. Amethod according to claim 7, wherein a temperature-time curve is used toregulate the melting of the solder layer.
 9. A method according to claim1, wherein the melting of the solder layer is produced by heating in afurnace.
 10. A method according to claim 1, wherein the melting of thesolder layer takes place under a protective gas.
 11. A semi-finishedproduct for use as a contact for an electrical switching device,comprising:at least one contact layer; and one solder layer; wherein thesolder layer is in intimate contact over its entire surface with asolder side of the contact and wherein the solder layer has thestructure of a melt without any mechanical treatment; and wherein thesolder layer has a rectangular shape with an equal or slightly smallersurface than the contact layer, and has. 8harp contours, a definedthickness and an even surface.
 12. A semi-finished product according toclaim 11, wherein the solder layer is on the solder side of the contactand is largely bubble-free or at least has no bubbles whose diametersare larger than a thickness of the solder layer.
 13. A semi-finishedproduct according to claim 11, wherein at least a side of the contactwhich is opposite the solder layer is free of solder.
 14. Asemi-finished product according to claim 13, wherein side surfaces ofthe contact are also free of solder.
 15. A semi-finished product for useas a contact for an electrical switching device, comprising:at least onecontact layer; and one solder layer; wherein the solder layer is inintimate contact with a solder side of the contact over its entiresurface and wherein the solder layer has the structure of a melt; andwherein a solder-free marginal area with a sharp contour is present onthe solder side of the contact.
 16. A semi-finished product according toclaim 11, wherein the solder layer is rectangular in shape.
 17. Asemi-finished product according to claim 11, wherein the contact has asintered structure and the solder layer has a structure of a melt.
 18. Asemi-finished product according to claim 11, wherein the contact is atwo-layer contact point.
 19. A semi-finished product according to claim18, wherein the contact has a silver-metal oxide (AgMeO) layer on thecontact side and a pure silver layer on the solder side.
 20. Asemi-finished product according to claim 19, wherein the pure silverlayer is dissolved only slightly and wherein silver dendrites arepresent for no more than one-third of the thickness of the solder layer.21. A semi-finished product according to claim 11, wherein the solderlayer is a hard solder based on copper and silver with a phosphoruscontent (so-called Silfos solder) and wherein the melt structureconsists of a ternary eutectic and mixed crystals rich in copper and/orsilver.
 22. A method according to claim 1, wherein the hard solder isapplied as a flat solder layer with a defined contour using a solderfoil or the like.
 23. A method according to claim 5, wherein thehigh-melting metal is tantalum (Ta), molybdenum (Mo), tungsten (W) ormixtures thereof.
 24. A method according to claim 5, wherein thehigh-melting metal is tantalum (Ta).
 25. A method according to claim 10,wherein the protective gas is a noble gas or nitrogen.
 26. Asemi-finished product according to claim 11, wherein the solder layer isrectangular in shape with a same size or smaller size surface as thecontact layer and wherein the surface is a flat surface.
 27. Asemi-finished product according to claim 11, wherein a solder-freemarginal area with a sharp contour is present on the solder side of thecontact.
 28. A semi-finished product according to claim 11, wherein thecontact has a sintered structure.
 29. A semi-finished product accordingto claim 11, wherein the contact is a contact such as a contact point,contact strip, or contact section.
 30. An arrangement for manufacturinga semi-finished product, the semi-finished product for use as a contactfor an electrical switching device, and including at least one contactlayer and one solder layer, wherein the solder layer is in intimatecontact with a solder side of the contact over its entire surface andwherein the solder layer has the structure of a melt, the arrangementincluding a device for melting a solder layer on a back side of thecontact, the melting device including means for covering a free surfaceof the solder layer.
 31. An arrangement for manufacturing asemi-finished product according to claim 30, wherein the means forcovering forms a molded body made of at least one of high-melting metaland ceramic.